I know this is an old thread, but I wanted to add a little bit of info of what I found on my star trek. Mine was a newer game with the screw already in the playfield as shown in other pictures, it would get stuck very often in that area, so I decided to try fixing it with the washers, and I also bought a nylon screw cap and I was going to try that.

I had a buddy over and we were working on the machine but he had to leave soon and that was the last thing I was going to do, I got the screw out but was having trouble getting it back in so we left it out and it seems to get stuck less. The action in the pop bumper area is notciably better, seem to get more pops, the ball will go back through the beam me up lanes more often, just much more lively, reminds me of the LE I played at texas pinball festival which I think was Zitts game.

It gets stuck less often, but when it gets stuck it takes more to get it out. It seems like with the screw in there if the ball was rolling slowly it had a chance to get stuck and it would, with the screw out it takes the ball moving fast, or hitting in just the right spot to get stuck, if not it will bounce of the rubber and keep moving down.

I am going to try the nylon screw cover next to see if that helps, but right now, play ability wise, I prefer no screw because of the improved pop bumper action.